1. Field of the Invention
This invention relates to improved apparatus for detecting an overheating condition in a gas-cooled dynamoelectric machine, and more particularly to an improved ion chamber apparatus for detecting the presence of submicron particles associated with overheating of parts in a gas-cooled turbine generator.
2. Description of the Prior Art
Large turbine generators are cooled by forcing a stream of a cooling fluid such as hydrogen over the heat producing parts of the machine. Because of the high flux densities present in the core of such a machine, localized overheating may cause degradation of the insulation disposed on the laminations and end turns. Because of the potentially catastrophic consequences of such degradation, it is desirable to provide an early warning of an overheating condition.
Apparatus such as the generator condition monitor has been developed to provide early warning of an overheating condition. The generator condition monitor is a highly sensitive device which rapidly detects the presence of particles in the hydrogen atmosphere of a turbine generator. Operation of the generator condition monitor is based on the principle that very high concentrations of submicron particles are produced whenever any material within the generator is heated sufficiently to initiate thermal decomposition. When an overheating situation arises within the generator, organic materials in the overheated area are affected first and degrade, producing particulates which enter the gas stream. In conventional practice, special organic compounds are deposited throughout the generator which particulate at much lower temperatures than most organic materials utilized in usual generator construction.
In one well known generator condition monitor, submicron particles are detected by their influence on the output current of an ion chamber which is arranged to collect the hydrogen ions which are produced by a low level radiation source in the hydrogen gas stream which carries the particles. In the absence of the particles, almost all of the hydrogen ions are collected, resulting in maximum output current of a magnitude determined by the strength of the radiation source and the ionization properties of the gas stream. With particles present, some ions combine with them. Because the particles are much larger than the ions, the mobility of the resultant charged particle is less, and relatively few are collected in the ion chamber. The result is a decrease in the output current of the ion chamber, this decrease being a function of the particle concentration and particle size. Such an arrangement is disclosed by Skala in U.S. Pat. No. 3,573,460. U.S. Pat. No. 3,427,880 issued to Grobel et al describes an application of such an ion chamber for detecting pyrolysis products resulting from overheating of generator insulation.
Although the apparatus described in the above-named patents have functioned adequately for the detection of overheating in certain generators, there have been occasions when more specificity and sensitivity would have been advantageous in determining the existence of a potentially damaging overheating condition. With improved specificity and sensitivity, an earlier warning of incipient failure within a generator can be achieved, which in turn would lead to an earlier shutdown of the machine and consequently less damage to existing insulation.